... upon NaN values

Make the histogram bin calculation NaN-safe, by mapping NaNs to 0.
Ideally, NaNs should probably not be counted at all, but since we
already count negative values as 0, and > 1 values as 1, we might
as well not pessimize performance over it, at least until we add
support for unbounded histograms.

At the same time, improve rounding in the bin calculation, so that
the result is more accurate.